If you they are also the son of your brother, he would be your nephew. Otherwise, there is no direct relationship, though referring to him as nephew (and you as aunt/uncle) would be the standard in most families.
Your sister's son will call you his uncle, if you are a man, or his aunt, if you are a woman.
You call him your nephew. (pronouced nes-few)
There is no actual blood relationship. You would typically introduce her in that way, My son-in-laws's sister. Many refer to her as the sister-in-law.
Your sister's son is your nephew. You are his aunt or uncle.
If your sister had a son, then he is your nephew.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your sister in law is your husband's sister or your brother's wife.
Your father's sister's son (or daughter) is your first cousin.
He would call her his aunt.
Newphew
Nephew
Your nephew.